This invention relates generally to apparatus for automated transfer of semiconductor wafers between processing stations and more particularly to a means for holding a plurality of wafers in vertical positions not only during such transfers but also during processing.
Attempts have been made to automate the handling and transfer of thin substrates during ion implantation, sputter coating and other processes both for preventing contamination, abrasion or damage to the substrates and for achieving a high throughput in terms of the number of substrates processed per unit time. In the transfer systems disclosed in U.S. Pat. No. 4,311,427 issued Jan. 19, 1982 to Coad et al and in U.S. patent application Ser. No. 381,085 filed May 24, 1982 by Hertel et al, for example, wafer carriers, or cassettes, are provided to hold a plurality of wafers in vertically facing alignment and to move horizontally past a loading station of a processing chamber. The cassette is essentially a rectangular frame having a pair of parallel side walls which stand facing each other and are grooved so that standard-sized disk-shaped wafers can be held edgewise. A vertically movable blade can pass through the cassette to engage a wafer from below and to move it upward into the processing chamber. The grooves on the side walls of the cassette are vertical and parallel to each other so that the wafer can slide along these grooves into and out of the cassette while maintaining its upright position.
The wafer transfer system described above is disadvantageous in that each cassette is designed for wafers of a fixed, or standard size. Cassettes for wafers of different sizes such as diameters of 100 mm, 125 mm and 150 mm are currently available commercially. It seems, however, that cassettes for holding wafers of diameters in the range of about 10 mm to 40 mm are not readily available. This means either that such wafers cannot be automatically processed in prior art systems or that cassettes specially designed for such sizes must be provided. It is preferable, however, to be able to process wafers of different diameters without the necessity of replacing the cassette. When wafers which are much smaller than the standard size are processed, furthermore, it is desirable to be able to process more than one wafer at a time in order to attain high throughput.
It is therefore an object of the present invention to provide a wafer transfer system adaptable to wafers of various sizes.
It is another object of the present invention to provide a multiple wafer holder which can be used with a cassette for wafers of the standard size.